With the development of an audio signal coding/decoding technology, more and more electronic equipments transmitting data via an audio interface are produced. For example, the electronic equipment may be connected with a mobile communication device (such as a mobile phone) via a loudspeaker interface (such as a headphone interface) of the mobile communication device for receiving an audio signal output from the mobile communication device.
Furthermore, with the popularization of a USB interface, a large number of electronic equipments are provided with the USB interface. Thus, how to transmit the audio signal between the mobile communication device and the electronic equipment via the loudspeaker interface of the mobile communication device and the USB interface of the electronic equipment is a problem which needs to be solved currently.
An adapter cable for connecting the loudspeaker interface and the USB interface (for example, the adapter cable for connecting a 3.5 mm headphone interface and the USB interface) in the prior art can only be connected with a USB master equipment (for example, a personal computer) and a loudspeaker interface equipment (for example, the mobile communication device), such that the loudspeaker interface equipment (for example, the mobile communication device) can communicate with the USB master equipment normally. However, the existing electronic equipments provided with the USB interface are generally USB slave equipments, which cannot communicate with the mobile communication device using the adapter cable for connecting the loudspeaker interface and the USB interface in the prior art.